A 3-kg slender rod rotates in a vertical plane about a pivot at B. A spring of constant k=300 N/m and of unstretched length 120 mm is attached to the rod as shown. Knowing that in the position shown the rod has an angular velocity of 4 rad/s clockwise, determine the angular velocity of the rod after it has rotated through (a) 90 degrees, (b) 180 degrees.